Produced water is underground formation water that is brought to the surface along with oil or gas. It is by far the largest (in volume) by-product or waste stream associated with oil and gas production. According to the American Petroleum Institute (API), about 18 billion barrels (bbl) of produced water were generated by U.S. onshore operations in 1995 (API 2000). Additional large volumes of produced water are generated at U.S. offshore wells and at thousands of wells in other countries, and it has been estimated that in 1999 there was an average of 210 million bbl of water produced each day worldwide. This volume represented about 77 billion bbl of produced water for the entire year. Given that worldwide oil production from conventional sources is nearly 80 million barrels per day (bbl/d, or bpd), one may conclude that 3 bbl of water are produced for each 1 bbl of oil worldwide, and that for the United States, one of the most mature petroleum provinces in the world, the ratio is closer to 6 or 7 bbl of water per 1 bbl of oil. One estimate, in 2004, calculated that more than 14 billion bbl of produced water was derived directly from state oil and gas agencies, with this estimate not including produced water from coal-bed methane (CBM) wells or from offshore U.S. production.
Management of produced water presents challenges and costs to operators. The cost of managing produced water after it is already lifted to the surface and separated from the oil or gas product can range from less than $0.01 to more than several dollars per barrel. If the entire process of lifting, treating, and reinjecting can be avoided, costs are likely to be reduced. With this idea in mind, during the 1990s, oil and gas industry engineers developed various technologies to separate oil or gas from water inside the well. The oil- or gas-rich stream is thereafter carried to the surface, while the water-rich stream is injected to an underground formation without ever being lifted to the surface. These devices are known as downhole oil/water separators (DOWS) and downhole gas/water separators (DGWS).
A number of downhole separation systems have been developed, tested and in some cases implemented, but these have been hampered by several problems implicit in the current systems. These problems include, for example, the fact that downhole equipment is more complicated and expensive that traditional equipment, the installation of the downhole equipment is more complex, and the downhole equipment has to be removed for maintenance at intervals using conventional and expensive equipment.
In addition, a number of authorities require metering of the water injected even if it is not brought to surface, meaning that the downhole equipment is further complicated. The pumps, and possibly meters, have to be powered and the data brought to surface. This requires installing cables into the well further complicating installation and removal, with these power and data cables themselves being sources of failure because they are exposed in installation and easily damaged. Finally, the application of downhole separation is usually most desirable in high water/low producing hydrocarbon wells which cannot stand the additional cost of the current technology.